Built between the mid-twelfth century and the last quarter of the thirteenth century on the ruins of the ancient Tabularium, since 1144 it was the seat of the Municipality of Rome and its highest representative: the Senator, responsible for the administration of justice and city interests.
The main facade of the thirteenth-century palace opened towards the future Piazza del Campidoglio with a series of loggias with round arches arranged on three levels. In the hall on the first floor, where the Senator's court was, the brick arches were marked by ancient recycled columns with Ionic capitals from the first quarter of the thirteenth century. A large decentralized arch on the right of the facade marked the main entrance from the square.
During the fourteenth century, the palace began to close like a fortress; many arcades of the loggias were buffered and towers were built by Boniface IX, then by Martino V (around 1427) and Niccolò V (1451), to reinforce the sides of the mighty building dominated by a fifteenth-century crenellated tower.
Michelangelo, between 1542 and 1554, designed the monumental double ramp staircase that serves the triple function of access to the new designed Senator's room, a plastic connection between the square and the palace and an elegant architectural setting for the two colossal ancient statues of the Nile and the Tiber. Some of his designs for the facade were later taken up by the architect Giacomo della Porta, who built and completed it only in the last decade of the sixteenth century. The ancient fifteenth-century tower was replaced by the grandiose bell tower built by the architect Martino Longhi il Vecchio between 1578 and 1582.